To be more helpful, will need more information about water parameters, age of tank, lighting. But based on my personal experience, the issue is caused by (some combination of): (1) nitrates/phosphates too high or too low, (2) lighting, (3) flow or (4) placement. Was it fully inflated for a long time after you introduced it to the tank? Or did this downhill slide occur shortly after you introduced it?
For me personally, my bubble spring to life when I moved it lower in the tank with less flow and less light.